When a semiconductor device such as a metal-oxide-semiconductor field-effect transistors (MOSFETs) is scaled down through various technology nodes, high-k dielectric material and metal are adopted to form a gate stack. In addition, a strained substrate using epitaxy (epi) silicon germanium (SiGe) may be used to enhance carrier mobility and improve device performance. However, current techniques to form these strained structures have not been satisfactory in all respects. For example, an etchant gas may be used with the epi process to maintain selectivity between the spacer and the substrate which can adversely impact the SiGe growth rate.